core

### Description of the Best Fit Alignment Dialog Box

Best Fit Alignment dialog box

The following features appear on all of the tabs in the Best Fit Alignment dialog box (Insert | Alignment | New | Best Fit button):

Available features list
The Available features list contains a list of all the features in your measurement routine before the alignment. For a full description, see "Dialog Box Description" and "Feature List box".

Alignment features list
The Alignment features area lists the features selected for the best fit calculation from the Available features list. You can sort each column by clicking on the column header. The check box to the left of each feature ID enables or disables the feature in the best fit calculation. If marked, the feature is used in the calculation; otherwise, it isn't.

If you select the Pause during execution check box (discussed below), the alignment is automatically reset to include all features in the best fit calculation during measurement routine execution.

If a calculated deviation is above the Deviation threshold value, the text for that feature is displayed in red.

You can modify weights by double-clicking the specific Weight factor for the feature in the Alignment Features list.

You can determine which axes are included in the calculation by double-clicking the specific item in the Include XYZ column for the feature. This displays the Include XYZ dialog box. Marking an axis includes the axis in the calculation. Clearing an axis excludes it.

You cannot change the Weight and Include XYZ settings of a scan's hits independent of their parent scan feature. Each hit uses the Weight and Include XYZ settings of its parent scan.

Deviation threshold box
The Deviation threshold sets the maximum acceptable value for the calculated deviation value. If a calculated deviation is above this threshold value, the text for that feature is displayed in red in the Alignment features area. The measurement units are the same as those defined in your system settings.

RMS Error
This displays the RMS Error value from the most recent alignment calculation.

Warnings area
The Warnings area displays any error messages from the most recent alignment calculation.

Compute button
The Compute button updates the best fit alignment calculation using the current features and settings.

Setup Tab

Setup tab

The Setup tab contains the most common settings for defining a best fit alignment. The following features are specific to this tab:

DOF (Degree of Freedom) area

DOF Check Boxes

The Tx, Ty, Tz, Rx, Ry, and Rz check boxes define which of the six degrees of freedom (rotation about X, Y, or Z axis, and translation in the X, Y, or Z direction) to include in the alignment. As the degrees of freedom are modified, the value of the Type list (2D, 3D, 3D No Rotation, and so on) is automatically updated according to the current settings. If the current combination of degrees of freedom does not match any of the pre-defined cases, the type is set to User Defined. Conversely, as the type is changed, the check boxes are updated to match.

Type

The 3D option causes the resulting alignment to be a 3D alignment that is both Translated and Rotated.

The 3D No Translation option causes the resulting alignment to be a 3D alignment that is Rotated but not Translated.

The 3D No Rotation option causes the resulting alignment to be a 3D alignment that is Translated but not Rotated.

The 2D option causes the resulting alignment to be a 2D alignment that is both Translated and Rotated in the selected 2D plane.

The 2D No Translation option causes the resulting alignment to be a 2D alignment that is Rotated but not Translated in the selected 2D plane.

The 2D No Rotation option causes the resulting alignment to be a 2D alignment that is Translated but not Rotated in the selected 2D plane.

2D Plane

The 2D Plane list sets the plane in which to calculate a 2D alignment.

If Type is set to User Defined, PC-DMIS automatically selects the Least squares option in the Best Fit Method area. The 2D plane list on this tab and the Rotate About options on the Advanced tab are then unavailable.

The 2D Plane list is available only if you choose one of the 2D options from the Type list. Available options are ZPLUS, XPLUS, YPLUS, ZMINUS, XMINUS, and YMINUS.

The coordinate image dynamically updates based on the marked vector or coordinate check boxes.

Best Fit Method area

The Best Fit Method area contains several methods that you can use to compute the best fit alignment. For information on these methods, see "The Best Fit Alignment Methods".

Iterate and repierce CAD area

Iterate and repierce CAD - If you select this check box, PC-DMIS performs an Iterative Best Fit alignment after you click OK. This pierces the CAD geometry and adjusts the feature nominals with each iteration using Tolerance and Max iterations to control the result. If this check box is cleared, PC-DMIS does a single Best Fit alignment.

Tolerance - Type a tolerance value. PC-DMIS uses this tolerance when searching for CAD surfaces to pierce. The measurement units for the tolerance is the same as those used by the measurement routine. The new nominal point becomes the CAD point closest to the actual feature, provided it is within this tolerance. If no CAD surface is found within this distance from the actual feature, then that feature is ignored in subsequent iterations.

Max iterations - This determines the maximum number of times the best fit alignment algorithm iterates.

If the repierce operation fails for all of the features included in the Best Fit alignment, PC-DMIS displays an error message in the Warnings area and aborts the alignment calculation.

Advanced Tab

Advanced tab

The Advanced tab contains specialized settings for several less-common conditions. The following are features specific to this tab:

Rotate About area
This area has two boxes that define the theoretical and measured center of rotation. You can click Set to automatically populate these fields with X, Y, and Z values from the feature selected in the Alignment features list, or you can manually type your own X, Y, and Z values.

The Theoretical box contains the theoretical center of rotation for 2D and 3D best fit alignments.

This Measured box contains the measured center of rotation for 2D and 3D best fit alignments.

The Clear button clears both fields.

Use scaling check box
The Use scaling check box becomes available for 2D or 3D alignments when you select the Least squares method. It is not available for alignments with specified constraints.

When you use scaling, PC-DMIS computes a transformation (rotation and translation) and a scale factor that optimally matches the nominal data to the scaled measured data. The scaled alignment also scales all of the measured data and subsequent measured features in the measurement routine, multiplying it by the computed scale factor.

Once scaling has been applied to the measured data and features in a measurement routine, it cannot be undone. Only mark Use scaling on in a single alignment in the measurement routine.

You may find this useful, for example, in compensating for expansion or contraction of a part due to temperature.

Create weights check box
If you select this check box, the software scales the values in the Weights column located in the Alignment features list by the tolerance values of the dimensions that control each feature.

If you clear this check box, the dimension tolerances are not factored into the calculation. Only the weights you defined are included.

Pause during execution check box
If you select this check box, the software causes the measurement routine to pause execution and display the Best Fit Alignment dialog box. This allows you to examine the magnitude of the error in the alignment solution, identify any features or scan hits that are bad, eliminate them from the alignment, recalculate, and then repeat that process until you are satisfied with the alignment. When you click OK, measurement routine execution resumes.

This option is only available for portable measurement devices. For non-portable machine types, it is hidden and ignored during measurement routine execution.

Transforms Tab

Transforms tab

The Transforms tab displays the alignment transforms from the most recent best fit alignment calculations. These transforms represent the cumulative state of the measurement routine up to and including this best fit alignment. The following sections are specific to this tab:

Machine to Part area - Displays the calculated Machine to Part transform values.

CAD to Part area - Displays the calculated CAD to Part transform values.

Machine to CAD area - Displays the calculated Machine to CAD transform values.